Ohm's Law & Power Calculator
Enter any two values — voltage, current, resistance, or power — and the other two are solved instantly from Ohm's law and the power equations.
The equations
Ohm's law links voltage, current, and resistance; the power equations add watts:
V = I × R · P = V × I · P = I²R · P = V² ÷ R
From any two known quantities, the remaining two are fully determined. This tool picks the correct pair of formulas for whichever two boxes you fill in.
Quick reference
| Known | Voltage | Current | Resistance | Power |
|---|---|---|---|---|
| V, I | — | — | V ÷ I | V × I |
| V, R | — | V ÷ R | — | V² ÷ R |
| V, P | — | P ÷ V | V² ÷ P | — |
| I, R | I × R | — | — | I²R |
| I, P | P ÷ I | — | P ÷ I² | — |
| R, P | √(P×R) | √(P ÷ R) | — | — |
Common questions
Does this work for AC circuits?
These equations are exact for DC and for purely resistive AC loads. For reactive AC loads (motors, transformers) you also need power factor and impedance — use RMS values and treat the result as apparent, not real, power.
What if I fill in more than two boxes?
The calculator uses the first two values you enter and shows what it derived, so you can spot an inconsistent entry.